Clubroot is a disease caused by Plasmodiophora brassicae which affects the Brassicaceae family of plants, including many important vegetable and broad acre crops. All members of the family Brassicaceae are thought to be potential hosts for Plasmodiophora brassicae (Dixon, 2009, J Plant Growth Regul 28: 194). Susceptible cultivated crops include all varieties of B. oleracea, the Occidental Cole vegetables (Brussels sprout, cabbages, calabrese/green broccoli, cauliflower, culinary and fodder kale, kohlrabi); B. rapa (syn. B. campestris) including turnip, turnip rape, sarson, and the enormous range of Oriental variants which provide leaf and root vegetables such as Brassica rapa var. pekinensis and B. rapa var. chinensis (Chinese cabbages); B. napus including swede (rutabaga), oil seed rape, and fodder rape; and seed, condiment (mustard), and vegetable crops derived from B. carinata, B. nigra, and B. juncea. Related genera such as radish (Raphanus), cruciferous weeds, for example, Sinapis, and decorative ornamentals including stocks (Matthiola spp) and wallflower (Cheiranthus cheiri) can be infected. The scientific model plant Arabidopsis is also susceptible (Dixon, 2009, supra).
Clubroot disease symptom development is characterized by the formation of club-shaped galls on the roots of affected plants. As a result, the nutrient and water uptake by infected roots is inhibited. Above-ground symptoms include wilting, stunting, yellowing and premature senescence (Hwang et al, 2012, Mol Plant Pathol 13: 105).
Clubroot disease is estimated to be present in approximately 10% of all areas where host plants are cultivated (Diederichsen et al, 2009, J Plant Growth Regul 28: 265). Clubroot has been largely a disease of vegetable crops in the last century. However, in 2003, 12 clubroot-infested commercial fields were found in the central part of the province of Alberta. Thereafter, the number of fields with confirmed clubroot infestations has increased steadily, and, by 2010, more than 560 fields (over 35 000 ha) in Alberta had been identified as being infested with P. brassicae (Hwang et al., 2012, supra). Yield losses of 80%-91% were reported in studies with canola grown on clubroot-infested fields in Quebec. Seed quality was also reduced significantly, with declines of 4.7%-6.1% in oil content and 13%-26% in 1000-seed weights (Hwang et al., 2012, supra).
Plant resistance is a powerful tool to combat clubroot disease. Breeding for clubroot resistance focuses today on Chinese cabbage (B. rapa spp. Pekinensis) in Japan and Korea, oilseed rape in Germany and Sweden, and several B. oleracea vegetables. Recently released resistant cultivars belong to three Brassica species: B. napus, B. oleracea, and B. rapa (Diederichsen et al., 2009, supra).
Resistant sources of the European fodder turnips (B. rapa ssp, rapifera) have been identified, which have been used to transfer the clubroot resistance genes to Chinese cabbage. At least three independent dominant genes, which confer differential (race-specific or vertical) resistance to particular pathotypes of P. brassicae, appear to be present in turnip genotypes (Piao et al., 2009, J Plant Growth Regul 28: 252). Eight possible clubroot resistance genes present in B. rapa have been identified through QTL mapping: CRa from resistant source ECD02, CRb from Gelria R, Crr1, Crr2 and Crr4 from Siloga, Crr3 from Milan White, and CRk and CRc from Debra. Crr1, Crr2, Crr3, Crr4 and CRc are mapped to chromosomes R8, R1, R3, R6 and R2, respectively. CRa, CRb and CRk with Crr3 are mapped on the same linkage group of R3, but they are not located in the same chromosome region, except for CRk and Crr3 (Piao et al., 2009, supra; Sakamoto et al., 2008, Theor Appl Genet 117:759).
In B. oleracea, completely resistant accessions have been rarely identified. The inheritance of the clubroot resistance in B. oleracea appears polygenic and controlled by many dominant alleles with predominance of additive effects of with incomplete dominance. It has also been suggested that one of the resistances studied is controlled by two complementary genes (Piao et al., 2009, supra). At least 22 QTLs have been found in B. oleracea, indicating a complex genetic basis of clubroot resistance in B. oleracea. As the different mapping studies used different clubroot resistance sources and different P. brassicae isolates, a comparison of these QTLs is not possible (Piao et al., 2009, supra).
Clubroot resistance has also been observed in several B. napus cultivars. At least 22 QTLs for clubroot resistance have been identified in B. napus. A major gene, Pb-Bn1, has been mapped onto linkage group DY4, and at least two additive QTLs have been identified on chromosomes DY4 and DY15, respectively. In addition, epistatic interactions between nine regions with or without additive effects have been located. A major gene and two recessive genes derived from ECD04 have been identified in double-haploid populations. In resynthesized B. napus developed by crossing cv. Böhmerwaldkohl (B. oleracea) and ECD-04 (B. rapa), nineteen QTLs expressing resistance to seven isolated were detected on eight chromosomes, four of which were closely linked to each other on chromosome N03, and three were linked on chromosome N08. Genes CRk and Crr3 are located in the similar region of PbBn-k-2, PbBn-1-1, and PbBn-01:60-1 on NO3. CRa and CRb are independent from them. PbBn-01.07-2, PbBn-1-2, and PbBn-a-1 are linked to BRMS088 on chromosome N08 in B. napus, which is also linked with Crr1 on R8 in B. rapa. PbBn-k-1 is located on chromosome N02. The QTLs located on N03 and N19 contribute strong effects and confer broad-spectrum resistance (Piao et al., 2009, supra; and Werner et al., 2008, Theor Appl Genet 116:363).
The CRa gene of Brassica rapa has been fine-mapped and a TIR-NBS-LRR gene has been identified as the CRa gene (Ueno et al., 2012, Plant Mol Biol 80: 621). The Crr1 gene has been mapped and isolated from the B. rapa European fodder turnip “Siloga”. Crr1a also encodes a TIR-NB-LRR disease resistance protein (Hatakeyama et al., 2013, PLOS one 8: e54745 and WO2012/039445).
The CRb gene from B. rapa has been fine-mapped to a 140 kb genomic region. In this region, in which fourteen functional proteins were predicted, amongst which a Rho family proteins and two TIR-NBS-LRR proteins, which could be candidate genes for CRb (Kato et al., 2013, Breeding Science 63: 116).
To increase the durability of clubroot-resistant cultivars, the combination of the different clubroot resistance genes into a single line will be an important means for breeding cultivars with resistance to a broader spectrum of physiological races. Therefore, in order to stack genes without linkage drag using marker-assisted selection and transgenic approaches, there remains a need to develop molecular markers linked to the clubroot resistance genes. This invention provides the sequence of a clubroot resistance locus from a resistant Brassica napus line, as herein after described in the different embodiments, examples and claims.